It would be desirable to have available devices that provide ISDN data transmission rates of 144 kbit/sec. over normal phone lines of up to 1.8 km in a PBX system. These devices would operate as a short loop `U` interface in typical PBX applications. To achieve this in a cost effective manner, the devices would be designed to transmit and detect bursts of alternate mark inversion (AMI) data packets sent in a time-compression multiplexed (TCM) or ping-pong method to achieve the same rate of data transmission as a standard `U` interface.
The design would be optimized by providing an autozero circuit to cancel the offset through the receive path of the device. Reducing the offset through the system provides additional noise immunity with respect to the mark level detectors. This helps insure good data transmission, especially at longer line lengths where the attenuation due to the phone lines results in the smallest AMI pulse in the application. The object of this design would be to provide devices with a naturally high offset (+/-350 mV) with the additional noise immunity needed to transmit data reliably, even in the presence of noise. Currently, high offset parts which may be good are typically scrapped rather than increasing test time to perform a bit error rate (BER) test on a tester, which is prohibitively expensive.
Therefore, it is an object of the present invention to provide an autozero system that dynamically cancels the offset in the receive path of the device. The autozero circuit handles the shifts in total receive path offset due to the shifts in both the coarse and the fine AGC gain settings.